Radioactive caesium and iodine has been deposited in northern Japan far from the Fukushima Daiichi nuclear plant, at levels that were considered highly contaminated after Chernobyl.

The readings were taken by the Japanese science ministry MEXT and reveal high levels of caesium-137 outside the 30-kilometre evacuation zone, mostly to the north-north-west.

Iodine-131, with a half-life of eight days, should disappear in a matter of weeks. The bigger worry concerns caesium-137, which has a half-life of 30 years and could pose a health threat for far longer. Just how serious that will be depends on where it lands, and whether remediation measures are possible.

The US Department of Energy has been surveying the area with an airborne gamma radiation detector. It reports that most of the "elevated readings" are within 40 kilometres of the plant, but that "an area of greater radiation extending north-west… may be of interest to public safety officials".Caesium contamination

An analysis of MEXT's data by New Scientist shows just how elevated the levels are. After the 1986 Chernobyl accident, the most highly contaminated areas were defined as those with over 1490 kilobequels (kBq) of caesium per square metre. Produce from soil with 550 kBq/m2 was destroyed.

People living within 30 kilometres of the plant have evacuated or been advised to stay indoors. Since 18 March, MEXT has repeatedly found caesium levels above 550 kBq/m2 in an area some 45 kilometres wide lying 30 to 50 kilometres north-west of the plant. The highest was 6400 kBq/m2, about 35 kilometres away, while caesium reached 1816 kBq/m2 in Nihonmatsu City and 1752 kBq/m2 in the town of Kawamata, where iodine-131 levels of up to 12,560 kBq/m2 have also been measured. "Some of the numbers are really high," says Gerhard Proehl, head of assessment and management of environmental releases of radiation at the International Atomic Energy Agency.Health risk

Whether people's health is at risk is not clear, however. Epidemiologists still argue over how many cancers were caused by caesium released by ChernobylMovie Camera. "We would investigate the exposures and effects when the emergency phase is over," says Malcolm Crick, secretary of the UN Scientific Committee on the Effects of Atomic Radiation.

It isn't a straightforward matter of how much caesium has landed. People's exposure depends on the local soil type, says Proehl. Sandy soil readily releases it, but clay binds it tightly, so contaminated clay can simply be buried.

Otherwise, it depends on whether the caesium lands on farms and gardens, or relatively undisturbed forests and mountains. With thousands of people in northern Japan made homeless by the tsunami, further evacuation of areas affected by the uncertain risk of fallout seems unlikely.